An aqueous polyurethane resin dispersion has been widely utilized as a starting material for a paint or a coating agent, for example, a starting material of a paint or a coating agent for coating an outside and inside of an aircraft, automobile, etc., an external wall surface and a floor material of a house, and the like. The above-mentioned coating films not only play beauty of appearances, but also take a role of protecting the substrate materials, so that hardness, strength and durability are required. For this purpose, an aqueous polyurethane resin dispersion which provides a harder coating film has been strongly desired.
In order to satisfy the above-mentioned properties, aqueous polyurethane resin dispersions using various kinds of polyols have been developed. A polyol is a useful compound as a starting material for preparing a polyurethane resin by the reaction with an isocyanate compound. For example, as a polymer polyol, a polyester polyol, polyether polyol, polycarbonate polyol, etc., have been used for the preparation of an aqueous polyurethane resin dispersion. Among the above-mentioned polymer polyols, the polycarbonate polyol has characteristics that it provides tough coating film as compared with those of the polyester polyol and polyether polyol. Of these, it has been known that the aqueous polyurethane resin dispersion prepared by using a polycarbonate diol having an alicyclic structure provides a coating film excellent in weather resistance and resistance to moist heat (see Patent Literature 1).
Also, it has been known that an aqueous polyurethane resin providing a coating film having high moisture permeability by providing a polyether group at the side chain using a specific polyol (see Patent Literatures 2 and 3).
However, the aqueous polyurethane resin dispersion using a polycarbonate polyol having an alicyclic structure is inferior in drying property, so that there is a problem that hardness is insufficient depending on the drying conditions. Also, there are problems that if the drying time is made long, the whole coating step takes longer times, and that if the drying temperature is made high, the substrate is affected by the temperature in some cases, etc.
Also, there are problems that in the aqueous polyurethane resin having a polyether group at the side chain has high moisture absorption and inferior in water resistance, high hygroscopicity, and inferior in water resistance.    [Patent Literature 1] JP H06-248046A    [Patent Literature 2] JP 2006-335951A    [Patent Literature 3] JP H01-104612A